The present invention relates to methods and apparatus for treating gas and, more particularly, for removing components from the gas.
Treatment of gas flows for the recovery or elimination of components present in a comparatively small quantity as gaseous compounds, liquid droplets or solid particles or particulate matter is a great problem in many industrial branches. Purification of stack gases from sulphur oxides, nitrogen oxides, hydrochloric acid, particulate matter, etc. is such a typical problem. Another typical example is the purification of synthesis gas from small amounts of impurities like hydrogen sulphide and carbonyl sulphide.
Many processes and apparatuses are described in the literature which utilize different principles of these treatments. Several of these methods are specific for special types of impurities, for instance particulate matter in stack gases, whereas other methods are of a more general nature. The present invention is of a completely general character and can be applied to many different fields with different embodiments within the frame of the invention.
Treatment of a gas flow in general takes place by bringing the incoming gas, i.e., the upstream flow, into contact with a material for treatment, i.e., agent. The agent has the property to take up the matter carried by the gas, which matter is to be eliminated from the gas flow, before it leaves the contact zone as down-stream flow. It is a characteristic feature of this invention that it gives very well defined contact conditions between the incoming gas and the agent, preferably in counter current, which permits as effective a treatment as possible with the least quantity of used up agent.
The present invention has certain points in common with the type of packed towers which are described in the French Pat. No. 1234396 where the packing is a uniform fluidized bed of light balls. The bed of balls is contained in an upper region of the tower where it is retained by means of a grid. The gas is blown through the liquid and the bed of balls whereby a large contact area is generated. Thanks to the strong movement in the bed of balls (which distinguishes this bed from the conventional packed tower) the composition of the liquid will be about the same in the whole active part of the tower.
Attempts have also been made to produce a counter-current flow between liquid and gas in a tower containing light balls of the same kind being used according to the procedure described in the French Pat. No. 1234396. According to the invention, described in the Swedish outlayed patent application 334 595, the balls, the so-called light weight elements, are kept in an uncontrolled movement in a contact zone along linear paths which are longer than any dimension of the elements whereby the volume of the contact zone is at least twice the volume taken up by the elements when they are at rest. This bed is expanded compared to what is described in the French patent referred to above so as to produce a more pronounced counter-current flow. There is, however, a stirring effect when it comes to the liquid phase because of the uncontrolled movement of the balls in the whole contact zone.
Svenska Flaktfabriken, a Swedish company, has developed an apparatus (Flakt Nateko Gasreaktor) which gives a well developed counter-current flow. The agent, in this case a solid material, is disposed on the surface of heavy balls which are being transported through a contact apparatus with decks. The balls are moved from deck to deck in counter-current relative to the gas flow. The agent is eliminated from the balls after the passage through the contact apparatus. The gas is passed through a packed bed of balls on each deck. It is not a long step from this apparatus to the conventional packed tower, which, however, does not permit the use of an agent in a solid state.
The simple scrubber is perhaps still a contactor so far unsurpassed with low pressure drop but it does not lend itself easily to counter-current contact in a simple way.
The processes and apparatuses described above as well as many other processes and means described in the patent literature and in reference books in this field like Pollution Technology Review No. 21, 2nd Ed., have the purpose to satisfy conflicting requirements. These requirements are in particular:
(a) Low pressure drop which is of particular importance in stack gas cleaning.
(b) Counter-current contact between gas and agent which gives a low quantity of impurities in the down-stream gas and an effective utilization of the agent.
(c) Almost complete consumption of the agent considering economic and subsequent processes, for instance deposition.
(d) Simple, inexpensive and reliable apparatuses.
The last mentioned requirement may seem trivial but is nevertheless of great importance. The cost for gas purification is frequently the heaviest item in the overall expense of many process systems. It is therefore as justified to try to develop better processes for gas purification as to develop the main processes.
The present invention satisfies the above requirements in a surprisingly simple and efficient manner. The invention can be used for many different applications within the technology of gas purification and gas treatment described in the publications referred to above, and gives thereby great advantages compared to the state of art. The invention gives primarily an effective and well controlled contact between the agent and the gas flow in a simple way. Thanks to this controlled contact the other requirements mentioned above are also satisfied.